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OCTOBER 2021 I DESIGN007 MAGAZINE 57 outer microstrip layer. However, if critical dig- ital signals are routed embedded between the planes, then this issue can be totally avoided. Key Points • e majority of electronic products today are assembled using the no-clean soldering process. • No-clean flux residue can lower the characteristic impedance of an intercon- nect, creating an alternative path to the signal. • One should consider the reliability of partially activated flux residues under component bodies and RF shields. • e effects of flux residues are measured in terms of their effective dielectric properties on microstrip structures. • Microstrip resonator structures are widely used for determining the dielectric constant of substrate materials at high frequency. • e presence of flux residue reduces the resonant frequency over the entire band- width but has its greatest impact at higher harmonics. • When the flux residue is deposited around a microstrip signal conductor it also con- tributes to the effective dielectric constant of the structure. • e dielectric constant of a laminate affects the characteristic impedance of a transmission line and the propagation delay of the line. • Low-loss materials can dramatically improve the propagation and quality of the signal at high frequencies. • e presence of flux residue tends to increase the dielectric constant, reducing the signal quality. • Unexpected changes in dielectric constant and impedance result in distortion to the higher-order harmonics making up a high-speed digital signal, with loss of digital signal integrity. DESIGN007 References 1. RF Characterization of No-clean Solder Flux Residues, Maeve Duffy, et al. 2. Selecting PCB Materials for High-Speed Digital Circuits, John Coonrod, Rogers Corp., Microwave Journal. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at www.icd.com.au. To read past columns or con- tact Olney, click here.